Modularly constructed heating elements for electric furnaces



United States Patent 'Of 3,506,771 MODULARLY CONSTRUCTED HEATING ELEMENTS FOR ELECTRIC FURNACES Stephen F. Cole, Jr., 26 Meadowbrook Lane, Chalfont, Pa. 18914 Filed Oct. 10, 1968, Ser. No. 766,451 Int. Cl. H05b 3/62 U.S. Cl. 13-25 10 Claims ABSTRACT OF THE DISCLOSURE The present device relates to an arrangement of heating elements for electrically heated furnaces. In the rpreferred embodiment the arrangement is composed of a plurality of rigid graphite members, some of which constitute support bars (which also act as electrical interconnection bars) and some of which constitute heating bars. Each of the heating bars is bolted across at least two support bars and at opposite ends thereof in order to form a continuous ohmic or electrical resistance path. The electrical power is connected to one or more of the supporting bars. The heating bars which very often wear out or burn out can be easily replaced by simply removing the bolts holding the heating bars to the support kbars and replacing the worn out or burned out heating bars with the new heating bar. In addition the heating bars are formed to provide large contact surfaces with the support bars, this (bolted) configuration diminishes the occasions for arcing and permits great exibility in altering the value at the ohmic path.

BACKGROUND Electrical furnaces, that is furnaces which are heated by electrical energy-are employed in great measure in the vacuum furnace art. In the vacuum furnace art items are heated while being held in a vacuum or under near vacu- -urn conditions. Vacuum furnaces are used for brazing metals; hardening metals; annealing metals; in general all forms of heat treating operations; degassing materials, such as ceramics; as well as for plating various products.

In one of the embodiments of these electrically heated vacuum-furnaces, -the heating elements are made of rigid graphite. Rigid graphite being formed of carbon which has been pressed and formed into a rigid mass capable of being shaped into various configurations, such as oblong at elements with apertures therein or bolts with threads thereon and the like. Rigid graphite is used because it is a good electrical resistance material, i.e., it will conduct electricity but will develop heat (PR) and when held in contact with other heating graphite elements will not bond together therewith. In other words many of the materials which might provide the proper electrical resistance and therefore generate heat in accordance with formula 12R are materials which become amalgamated or have their molecules dilfuse one into another or in essence weld together when they are subjected to the kinds of heat which are necessary to make the vacuum furnaces effective. As indicated above .graphite does not diffuse, bond or weld together and hence is a very preferable material for an easily replaceable heating element. In addition rigid graphite has the quality of increasing its structural strength as it heats up.

In the past the graphite heating elements in such a furnace have been composed of elements which are entirely cut from one piece of rigid graphite. Obviously when these heating elements burned out or became broken for any reason the entire element had to be discarded. Improvement of the one piece element was effected by creating a continuous path made up of cylindrical heating rods which were press fitted into support members. Both 3,506,771 Patented Apr. 14, 1970 rice the support members and these cylindrical rods were/ and are made of rigid graphite. The rods are press fitted into the one end of one support member and into an opposite end of another support member to form a continuous ohmic path by which to generate the heat. In one version of the cylindrical heating rod, the rod is actually made hollow and the hollow rod is press fitted into a circular hole in the support member. Originally the cylindrical heating rods were tapered at the end and press tted in accordance with a wedged surface principle. The cylindrical heating rod, elements, while theoretically being meritorious for purposes of easily removing the burned out or -worn out or broken heating elements, have failed in many respects in this regard.

What actually has happened with the cylindrical heating rod elements is that the pressure -of the rod against or within the aperture at the holding bar (to etfeot the press fit) often causes the holding bar to rupture and crack at the aperture location. In addition if the press tit effected is not tight enough as electrical current passes between a support member and the cylindrical heating rod element, there is great arcing at the surface between the inner surface of the aperture in the holding bar and the portion of the rod which is press fitted into the aperture at the holding bar or support member. The arcing very often occurs, initially, because the lapping or forming of the cylindrical rod does not provide a true circular circumferential surface or if the t is not tight enough. When the arcing has once occurred the two pieces involved are no longer useful. Further the cylindrical rods have provided a problem in that the press tit very often is so tight that the dislodging of the rod from the holder element ends up in a situation where the holder element Igets broken. In addition the cylindrical heating rod provides very little flexibility with respect to readily altering the value of the ohmic path and does not perform with the optimum eiiiciency.

SUMMARY The present device offers a vast improvement over the cylindrical heating rod modular concept in that the flat heating bars of the present invention efrect a good electrical connection between the large at surfaces on the heating bars and the supporting bars and are simply held in abutment by bolts made of graphite. When one of the present heating bars burns out or gets broken for whatever reason, then it is a simple matter to unscrew the bolts from the support bar, remove the broken or burned out heating bar and replace it with another (new or whole) heating bar. Other advantages of the present heating bar over the cylindrical rod heating bar will become apparent in accordance 4with the description given below.

The foregoing features of this invention will be best understood by reference to the following description of the invention taken in conjunction with the accompanying drawings, wherein FIGURE 1 is a pictorial schematic showing the cylindrical rods of the prior art heating elements;

FIGURE 2 is a pictorial schematic showing the flat heating bars bolted together with support bars in accordance with the present invention;

FIGURES 3A and 3B are embodiments of the heating bar of the present invention which are different from the embodiment shown in FIGURE 2;

FIGURE 4 is another embodiment of the heating element of the present invention which is different from the heating elements shown in any of FIGURES 3A and 3B or FIGURE 4;

FIGURE 5 is a schematic showing the power connections to an arrangement of heating elements in accordance with the present invention; and

FIGURE 6 is another schematic showing the electrical connections for heating elements in accordance with the present invention.

|Consider FIGURE l which shows a pictorial schematic of the heating element arrangement for a furnace, as used in the prior art. In FIGURE l there are shown three holding bars 11, 13 and 15. It will be noted that each of the holding bars as shown in FIGURE 1 has two apertures formed therein such as the aperture 17. In between the aperture 19 and 21 there is inserted a cylindical heating rod element 23. In the embodiment shown, the heating element 23 has a key collar or protrusion 25 which determines the limit of the insertion of the rod 23 into the holding bar 15. A key collar or protrusion on the opposite end of the cylindrical heating element 23 limits the depth of the insertion of the rod 23 to the holding element 13.

A similar cylindrical heating rod 27 is inserted between the apertures 29 and 31 and no detail discussion thereof need be given. Part of a heating rod 33 is shown inserted into the aperture 35 of the holding element 1I and this is for the purpose of indicating that there can be a continuation .of this zig-zag pattern, or continuous ohmic path, between the end 37 of the holding element 15 (or in the alternative a heating element rod inserted into the aperture 17) and the end of the rod 33 or whatever other elements may be connected thereto.

Connected to the holding element 13 is shown a power rod 39. Electrical power is supplied to the power rod 39 and is transmitted along the ohmic path, or paths, just described to an electrical circuit return element whose purpose is to complete the electrical circuit. The electrical arrangement can be quite similar to the schematics shown for the present invention in FIGURES and 6.

vNow it becomes a problem when one or more of the heating elements, such as elements 23, 27 or 33, is burned out, to remove that element without cracking the holding elements 11, 13 or 15 for a number of reasons. For instance consider that the element 23 is burned out and the element 23 has to be removed from the holding element 13 at the aperture 21 end. When the holding element 13 is separated from the heating element 23, the movement of the holding element 13 around the fulcrum point at the aperture 31 has often caused the holding element 13 to crack or break at the aperture 31. In the alternative or simultaneously the press t between the cylindrical rod 23 and the aperture 21 is often such that in the course of trying to dislodge the rod 23 from the holder 13 the holding element 13 cracks at the aperture 21 or when the rod gives way, the force thereof drives the end of the rod toward aperture 19 with such force that the holding element cracks at the aperture 19.

In addition the arrangement shown in FIGURE 1 has other limitations. For instance it is very dicult to get maximum contact between the cylindrical heating element 23 and other such heating rods and the holding elements into which they are press tted. Accordingly there very often is arcing occurring between the outer surface of the heating element 23 and the inner surface of the apertures into which they are press fitted. Further the heat generated within the rod is less eliciently radiated to the ambient, which is the chamber of the furnace, and hence the eliciency of the rod insofar as the amount of heat it might produce is greatly diminished. In addition to the foregoing if it is the desire of the designer, or the user, for whatever reasons (and the reasons are many) that the heating rod should have increased electrical resistance or diminished electrical resistance for whatever the particular job that the furnace is used to accomplish, the prior art heating element arrangement shown in FIG- URE 1 does not provide an easy method to alter the generation of heat thereby.

FIGURE 2 shows a pictorial schematic of the present invention. In FIGURE 2 there are shown three holding bars 41, 43 and 45. Bolted to the three holding bars just mentioned are heating bars 47, 49 and 51. The heating bar 47 is shown in part and is analogous to the heating bar 33 in FIGURE l. The heating bar 47 may be connected to another holding bar which in turn may be connected to another heating bar and so forth, so that the pattern may continue. The heating -bar 47 is connected to the holding bar 41 by virtue of the bolt 53. The heating bar 47, the bolt 53 and the holding bar 41 are all made of rigid graphite.

In the preferred embodiment the graphite is obtained from the Union Carbide Company, and is designated as CS graphite. Other forms of commercially designated graphite such as AGSX graphite, AGSG graphite, ATL graphite are obtainable from Union Carbide Company and can be used.

The heating bar 49 is connected to the holding bar 43 and to the holding bar 41 by virtue of the bolts 55 and 57 and as was just mentioned the heating bar 49 as well as the bolts '57 and 55 and the holding bar 43 are all made of rigid graphite. The heating bar 51 is connected between the holding bar 43 and the holding bar 4'5 by virtue of the bolts 59 and 61, and the bolts 59 and 61 as well as the holding bar 45 are also made of rigid graphite.

In FIGURE 2 the holding bar 45 is shown cut away so that the threads 63 of the aperture 65 can be shown. In addition the bolt 67 with its thread 69 is shown in juxtaposition to the aperture 65. It should be understood that other rigid graphite heating elements can be connected to the holding bar 45 iby virtue of the bolt 67 and the other heating elements can be arranged with holding bars to continue the ohmic path shown. It should be further understood that the heating elements can -be threaded and the holding bars unthreaded and heating elements can be arranged from both the top and the bottom of the holding bar such as depicted by the phantom bar 71. In addition it should be understood that each of the bolts 53, y57, 55, l59 and 61 is threaded, similar to bolt 67, while the apertures into which they tit are threaded similarly to the aperture 65.

Connected to the holding bar 43 is an electrical power bar 73 which is connected thereto by a threaded (rigid) graphite bolt 75. The electrical power bar 73 provides electrical current to the ohmic path, or paths as the case may be, and it should be understood that there are other electrical connections to the path so that the electrical current may be returned It should be Well understood that when an electrical current passes along the heating bars 47, 49, 51 or 71 these bars become hot in accordance with the mathematical concept 12R (watts) and transmit heat to the chamber of the furnace.

As was mentioned earlier it often becomes desirable to change the form of the ohmic path to generate either more or less heat depending upon the operation in which the furnace is being used. FIGURES 3A and 3B show two configurations which will change the ohmic value of the heating bars. In FIGURE 3A the heating bar 77 has a narrowed center portion and therefore would generate less heat than for instance the heating bars shown in FIGURE 2 for a fixed applied voltage thereacross. On the other hand heating ibar 79 shown in FIGURE 3B has a wider path and therefore would generate more 'he-:at than the heating bars shown in FIGURE 2 for a fixed applied voltage thereacross. On the other hand for a fixed resistance in an overall heating circuit these elements Would have the opposite effect.

FIGURE 4 shows a heating bar `81 in which there are cut a plurality of slots 83. The slots 83 of course lengthen the ohmic path for the same physical distance between the apertures 85, 87 and thereby provide a greater amount of electrical resistance for the physical distance between two holding bars such as the holding bars 43 and 45 of FIG- URE 2. In this way less heat can be generated than would be generated by the bar 51 for a -ixed applied voltage thereacross. It `becomes apparent that the flat heating bars of the present invention enable changes to be easily made in ohmic path configurations and the simple dismantling thereof provides a very fiexible modular system which is not attainable in the prior art.

FIGURE shows electrical schematics for an arrangement of rigid graphite bars. The holding bars 89 through 95 and 123, 124 and 125 are connected in a scheme similar to that shown in FIGURE 2 by the heating bars 96 through 101 and 120, 121 and 122. Connected to the holding bar 89 and the holding bar 95 as Well as the holding bar 92 is an electrical power system made up of the power bars 103, 104 and 102. Connected to the power bars is a three phase electrical power generator 105 which supplies three phase power to the system. The electrical connection 126 completed the delta connection for the applied three phase power. Obviously the electrical circuit could be connected some other way. For instance in FIG- URE 6 there is shown an arrangement vof elements comprising the holding bars 106 through 110 connected together by :bolts and fiat heating bars 111 through 114 in an arrangement similar to that shown in FIGURE 2. In effect there is a complete ohmic path from the holding bar 106 to the holding bar 110. Connected to the ohmic path just described is a first electrical power bar 115 and a second electrical power bar 116. Connected to the electrical power bars 115 and 116 is a single phase power source or generator 117. Obviously other arrangements could be made to supply electrical power to this system for the purpose of generating heat in the heating bars 111 through 114.

The present invention provides fiexibility in that many arrangements of ohmic paths can be arranged such as those shown in FIGURES 3A, 3B and 4 as well as that shown in FIGURE 2 and even others which are not shown but which come within the spirit of the invention, if their purpose is to change the value of the ohmic path and yet be readily adaptable to the system. The present system also provides an easy method for disassembling and therefore replacing the heating elements when the heating elements become burned out or worn out for whatever reason. In addition the present system provides means for reducing and virtually eliminating any arcing between the heating elements and the holding bars by virtue of the large surface connection between the holding bars and the heating elements due to the fiat surface connection and the capacity to enable the user to tighten the heating element against the holding bar by turning the bolts. In addition the present system has an advantage over the prior art in that the efficiency of the heating element is greatly enhanced because of the amount of surface that is available to conduct and radiate heat into the ambient, i.e. the chamber of the furnace with which it is used. This feature increases the radiation efficiency and lowers the surface watts density on theheating element itself for a given length.

i The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A heating element arrangement for a furnace which is heated by electrical energy comprising in combination: (a) a plurality of first graphite elements, each of said first graphite elements being formed substantially fiat and elongated and having first and second ends thereof; (b) first and second apertures formed respectively in said first and second ends of each of said first graphite elements; (c) a plurality of second graphite elements, each of said second graphite elements having first-and second fiat surface areas thereon, each of said first and second Ifiat surface areas respectively having an aperture formed therein; (d) a plurality of first and second graphite securing means, each of said first graphite securing means disposed with a different one of said first graphite elements to secure the first end thereof with a different one of said second graphite elements at said second fiat surface area thereof, and each of said second graphite securing means disposed with a different one of said first graphite elements to secure said second end thereof with a different one of said second graphite elements at said first flat surface area thereof; and (e) means to be connected to electrical power source means, formed with selected ones of said graphite elements.

2. A heating element arrangement according to claim 1 wherein said first graphite elements are rectangularly shaped and wherein said second graphite elements are rectangularly shaped.

3. A heating element arrangement according to claim 1 wherein said first and second graphite securing means are threaded bolts and wherein said apertures formed in said second graphite elements are threaded apertures formed to match the threads of the said graphite bolts.

4. A heating element arrangement according to claim 1 wherein said first graphite elements have slots cut therein in order to increase the resistance values thereof.

5. A heating element arrangement according to claim 1 wherein there is further included in a three-phase power supply connected to particular ones of said second graphite elements.

6. A heating element arrangement according to claim 1 wherein there is a single-phase electrical power supply connected to particular ones of said second graphite elements.

7. A heating element arrangement according to claim 1 wherein said first graphite elements are formed to be substantially narrow to increase the electrical resistance thereof.

8. A heating element arrangement according to claim 1 wherein said first graphite elements are formed to be substantially wide to decrease the electrical resistance thereof.

9. A heating element arrangement according to claim 1 wherein said second graphite elements have third and fourth fiat surface areas and wherein additional first graphite elements are disposed to be secured by said first graphite securing means to different ones of said third fiat surface areas at the first ends of the respective additional first graphite elements and further disposed to be secured by said second graphite securing means to different ones of said fourth fiat surface areas at said second ends of the respective additional graphite elements.

-10. A heating element arrangement according to claim 1 wherein said means to be connected to electrical power source means includes third graphite elements and third graphite securing means formed and disposed such that said third graphite elements are secured to selected ones of said second graphite elements.

References Cited UNITED STATES PATENTS 1,472,139 10/1923 Reid 13-25 1,572,881 2/1926 Brace l3-25 1,622,621 3/1927 Woodson 13-25 2,009,732 7/1935 Harper et al. 13--25 3,395,241 7/1968 Roman 13-25 H. B. GILSON, Primary Examiner 

